The market is inundated with optical disk storage devices. They are used to store audio information, such as in Compact Disk (CD) players, as well as audio, visual, and computer information, such as in CD-ROM and the more recent Digital Video Disk (DVD) players. The information is typically recorded as a binary sequence by writing a series of "pits" on the optical medium which represent binary "1" and "0" bits. During a read operation, the disk is irradiated by a laser beam emanating from a read head (transducer) positioned over the disk. The pits alter a characteristic of the laser beam (e.g., the phase) such that the reflected beam is modulated by the originally recorded data sequence. The reflected beam is converted into an analog read signal, referred to as an RF read signal due to its high frequency, which is then demodulated by read channel circuitry to reproduce the recorded data.
The mechanics of the read head include a laser diode for generating the laser beam, an objective lens for focusing the laser beam onto the disk, and optics for detecting the laser beam reflecting off the disk. This bulky configuration makes optical disk storage devices less useful as random access devices because of the slow access time in positioning the read head over a particular part of the disk. The entire sled assembly 2 containing the read head is positioned radially over the disk 4 by means of a lead screw 6 as shown in FIG. 1. The frequency response of this servo system is much slower as compared to magnetic disk storage devices. Consequently, optical disk storage systems are better suited for storing large blocks of contiguous information, such as long streams of audio or visual data, where the number of random access seeks to different locations on the disk is minimized.
Because of the contiguous character of the data, it is typically recorded in data tracks that spiral inward from the outer diameter to the inner diameter of the disk 4 as shown in FIG. 1. This format maximizes the data capacity by allowing for a constant recording density (linear data density) from the outer to inner diameter of the disk 4. However, because it is desirable to maintain a constant data rate consistent with the rate of the audio or visual data stream, the rotation speed of the disk is varied to maintain a constant linear velocity (CLV) as the read head traverses radially over the disk. Thus, an optical disk storage device typically comprises a variable speed spindle motor 8 as shown in FIG. 1 which is adjusted by a CLV servo loop 10 to maintain a constant data rate relative to the position of the read head 2. The recorded data typically includes special fields referred to as servo address marks (SAM) recorded at a predetermined interval within the data tracks. The SAM fields are detected by a read channel 12 and communicated to the servo controller 10. The frequency of the detected SAM fields is proportional to the linear velocity of the disk at the radial position of the read head; the speed of the spindle motor 8 is controlled to maintain a constant SAM frequency and thus a constant linear velocity.
Although random access seeks to different radial locations over the disk are less frequent due to the contiguous character of the data, they are still necessary. For example, it may be necessary to interrupt accessing an audio or visual stream to access related computer data located on a different track at a different radial location of the disk. In such instances, it is necessary to adjust the spindle speed to account for the corresponding change in the data rate. This is normally accomplished by making a course adjustment to the spindle motor during the seek operation as the read head traverses radially over the disk toward the target track, and then by making a fine adjustment based on the frequency of the detected SAM fields when the read head reaches the target track (or a track nearby). A drawback associated with the way timing recovery is implemented in prior art optical storage systems prevents any data from being read at the end of the seek operation until the CLV servo loop acquires the correct spindle speed. This added latency increases the overall access time of the storage system.
The function of timing recovery in an optical storage system is to determine the instantaneous baud rate of the data in the analog read signal so that the data can be accurately extracted. For instance, in the prior art optical storage device shown in FIG. 2, a timing recovery circuit 14 processes the RF read signal 16 to generate a data clock 18 synchronous with the baud rate. The data clock 18 is applied to an RF demodulator 20 which detects whether zero crossings in the RF read signal 16 occur during each data clock period. The presence of a zero crossing during a data clock period is detected as a binary "1" bit, and the absence of a zero crossing during a data clock period is detected as a binary "0" bit. The timing recovery circuit 14 adjusts the data clock 18 so that the detected zero crossings occur at the center of the data clock periods on average. Because adjustments to the timing recovery PLL are made only when zero crossings are detected, the recorded data stream is encoded so as to limit the maximum number of consecutive "0" bits. The recorded data stream is also encoded to ensure a minimum spacing between consecutive zero crossings which reduces the undesirable affect of intersymbol interference (ISI). The channel code which implements these run-length constraints is referred to as an eight-to-fourteen or EFM code because it encodes eight input bits into fourteen output bits such that the recorded data stream comprises not less than two consecutive "0" bits and not more than ten consecutive "0" bits.
An implementation of the prior art timing recovery circuit 14 is shown in FIG. 3. This circuit implements a phase-lock-loop (PLL) using a variable frequency oscillator (VFO) 22. A frequency synthesizer 24 generates a center frequency control signal 26 which sets the center frequency of the VFO 22 at the optimal data rate. A frequency/phase error between the actual and optimal baud rate is computed and added to the center frequency control signal 26 at adder 28 in order to lock the output of the VFO 22 to the actual baud rate of the RF read signal 16. A phase error detector 30 computes a phase error 32 between the RF read signal 16 and the data clock 18 output by the VFO 22. The phase error 32 is then filtered by a loop filter 34, the output of which eventually settles to a frequency offset 36 between the baud rate of the RF read signal 16 and the data clock 18. The frequency offset 36, together with the center frequency control signal 26, adjust the output of the VFO 22 until the data clock 18 is locked onto the baud rate.
The reason the optical storage system must wait for the CLV servo loop to acquire the target linear velocity for the spindle motor at the end of a seek operation is because the PLL in the timing recovery circuit 14 of FIG. 3 has a relatively narrow acquisition range. In other words, the initial baud rate in the RF read signal 16 must be close enough to the optimal frequency or the timing recovery PLL will not be able to acquire the baud rate.
There is, therefore, a need for an improved timing recovery circuit for use with optical storage devices that is capable of acquiring the baud rate immediately following a seek operation without having to wait for the CLV servo loop to acquire the correct spindle speed. An enabling aspect of the present invention is to provide a simple circuit for estimating the instantaneous baud rate at the end of a seek operation in order to accurately initialize the timing recovery circuit prior to reading the recorded data.